Black Holes

Throw a ball upward and it comes back down. Throw it faster and it goes higher. Throw it at about 11 kilometres per second and it never returns: that is Earth's escape velocity. Now imagine squeezing the Earth smaller and smaller without removing any mass. Its surface gravity rises, and so does the escape velocity. Squeeze it to the size of a marble and the escape velocity reaches the speed of light. Nothing can go faster than light, so nothing can leave. You have made a black hole.

The point of no return

Every black hole has an invisible boundary called the event horizon. Outside it you can still escape, with enough effort. Cross it and there is no coming back, in any direction, at any speed. The horizon is not a surface you could touch. It is simply the distance at which escape becomes impossible. For a black hole with the mass of the Sun that distance is about three kilometres. For the giant in galaxy M87 it is wider than our whole solar system.

Inside the horizon, as far as our equations can tell, all the mass is concentrated at a single point of zero size and infinite density called the singularity. Most physicists suspect the singularity is a sign that our theory of gravity is incomplete, not a real thing. What actually sits at the centre is one of the great open questions in science.

Where they come from

Most black holes are the corpses of massive stars. When a star more than about twenty times the Sun's mass runs out of fuel, its core collapses and nothing can stop it. The result is a black hole a few times heavier than the Sun. Our galaxy probably contains a hundred million of them, almost all invisible.

At the centre of nearly every large galaxy sits a far bigger kind: a supermassive black hole, millions or billions of times the Sun's mass. Ours is called Sagittarius A*, and it weighs about four million Suns. Astronomers tracked stars whipping around it at thousands of kilometres per second for decades before anyone could see it directly. How such monsters grew so large so early in the universe's history is still not fully understood.

Seeing the invisible

By definition a black hole emits no light, but its surroundings can blaze. Gas falling toward the horizon forms a spinning disc, heated by friction to millions of degrees, glowing in X-rays. Some black holes launch jets of particles at close to light speed that stretch for thousands of light-years.

In April 2019 the Event Horizon Telescope, a network of radio dishes across the planet working as one Earth-sized instrument, released the first image of a black hole: a glowing ring of hot gas around a dark shadow at the heart of galaxy M87, 55 million light-years away. Three years later the same team imaged Sagittarius A*. The shadows were exactly the size that Einstein's equations predicted a century earlier.

Time and gravity

Einstein's general relativity says that gravity is not a pull but a bending of space and time. Near a black hole that bending becomes extreme. A clock near the horizon ticks more slowly than one far away. If you watched a friend fall toward a black hole, you would see them slow down, redden and fade, never quite reaching the horizon. From their own point of view they would cross it in moments, noticing nothing special at the boundary itself, though the tidal forces of a small black hole would stretch them apart long before.

Despite their reputation, black holes do not suck. At a distance, a black hole's gravity is exactly that of any other object with the same mass. If the Sun were replaced by a black hole of equal mass, the Earth would keep orbiting just as it does now, in the dark.

What we still don't know

In 1974 Stephen Hawking showed that black holes should glow very faintly and slowly evaporate, through a quantum effect now called Hawking radiation. For a stellar black hole this would take vastly longer than the current age of the universe, so it has never been observed. But it raises a puzzle that has troubled physicists for fifty years: if a black hole swallows information and then evaporates away, where does the information go? Quantum mechanics says it cannot be destroyed. Resolving that paradox may require a new theory that unites gravity with the quantum world, something nobody has yet achieved.